1. Field of the Invention
This invention relates to a magnetoresistive type thin film magnetic head which is suitable for reproducing signal recorded on a perpendicular magnetic recording medium.
2. Description of the Prior Art
It is known that perpendicular magnetic recording is inherently more suitable for high density recording than longitudinal magnetic recording. However, the former has been involved in various problems in the reproduction operation. For example, for the reproduction with a wire-wound magnetic head, there have been proposed single pole heads and ring heads. But when a ring head is used to reproduce short-wavelength signals that characterize the perpendicular recording, the head needs to have an extremely small gap length, resulting in a very low efficient magnetic circuit of the head. Moreover, if turns of winding are increased in order to enhance the sensitivity of reproduction, the self resonance frequency will fall due to the increased head inductance. On the other hand, the signal frequency rises with decrease in recording wavelength, and a fall in the self resonance frequency of the magnetic head is extremely disadvantageous in reproducing signals. A single pole head cannot avoid the same problems of the wire-wound magnetic head.
The more serious common problem of electromagnetic induction heads is a small reproduction output voltage when the recording medium is fed slow relative to the head, and thus increased turns of winding for counter-acting the matter. On the other hand, in a multiple track arrangement where several magnetic heads are aligned, the wiring space poses a problem. In addition, when the thin film technology is used to fabricate wire-wound magnetic heads, the number of turns of winding is restricted, and a high-sensitivity magnetic head cannot be achieved.
In order to overcome the aforementioned problems, attention is paid recently to a magnetic head utilizing the magneto-resistive effect hereinunder ("MR"). One example of the conventional MR head is of a single element type, in which a strip of magneto-resistive element is placed perpendicularly to the recording medium with a current conducted in the longitudinal direction of the element and the magnetic flux of the recording signal is incident on the element surface at right angles with respect to the longitudinal direction. This type of magneto-resistive head is known to have the structure-dependent wavelength response characteristics solely determined by the width W of the magneto-resistive element. In order for the magneto-resistive head to reduce the wavelength loss sufficiently, the element width W needs to be made as small as the recording wavelength .lambda., and this is extremely disadvantageous for a head designed to handle short wavelength signals.
On the other hand, there is known a shielded MR head in which a high permeability material is placed on both planes of the MR element. This type of MR head has substantially the same wavelength response characteristics as of the conventional wire-wound ring head, and is known to be operable sensitively down to a range of relatively short wavelengths. In this structure, however, the magnetical and electrical insulation is needed between the MR element and the high permeability material, and the thicknesses g1 and g2 of the insulation layers on both sides correspond to the gap length of the conventional wire-wound ring head. In addition, in order to sufficiently reduce the gap loss for short wavelengths, which is approximately the product of the gap loss of g1 and the gap loss of g2, both the g1 and g2 must be made extremely small. It is very difficult in such circumstances to form a small gap length without magnetical and electrical leakage.